Depending on how oddly shaped or how large you want your room will determine how many actual rooms you'll need to create your large/free room. For the purpose of this tutorial, I just made a large rectangular room with several rooms I had in storage. You do not need to use these same rooms to create a larger/free room.
All is left to do with the new extra room is to add floor tile, wallpaper, and furniture.
I know in the past there was a problem with weeds and/or mushrooms growing in the extra room. I am not sure if they fixed that bug.